1. Field of Invention
This invention relates to the development of flexographic (photopolymer) printing plates using an improved developer solvent. More specifically, this invention relates to the use of diisopropylbenzene alone, or in combination with a co-solvent(s), as a washout solvent for the non-crosslinked polymer material in the printing plates to develop the relief image. Diisopropylbenzene is non-toxic, can develop plates quickly, causes minimal plate swelling and distortion, dries quickly, is easy to reclaim by distillation, has a very mild odor, has a reasonably low cost, is thermally stable, and works on virtually all known solvent-developed photopolymer plate types.
Ease of recycling of flexographic developer solvents is important to reduce waste, and because it is often more cost-effective to recycle than to continually replace used developer solvent with new. In this regard, azeotropic or azeotrope-like compositions that exhibit reduced boiling points, enhanced recovery rates and limited fractionation upon distillation are desirable. Thus, there is a need for an environmentally-friendly, easily recyclable, preferably azeotropic or azeotrope-like, low vapor pressure, high flash point, solvent that effectively develops flexographic printing plates. The present invention addresses this need.
The diisopropylbenzene-based solvents show selective solubility for non-crosslinked photopolymer material vs. crosslinked photopolymer material when compared to solvents of the prior art. This results in a reduction of plate swelling, image distortion and drying times.
2. Description of Prior Art
Flexography is a type of relief printing that uses flexible sheets of photopolymer to transfer an image onto a substrate. In the flexography process, photopolymer sheet is exposed to light through a negative or mask that blocks selected portions of the photopolymer from the light. The portions of the photopolymer that are exposed to light crosslink. The non-crosslinked photopolymer is soluble in many organic solvents, whereas the crosslinked photopolymer is harder and much more resistant to chemical solvent attack. By "washing" the selectively-exposed photopolymer sheet in a solvent bath, the unexposed portions of the plate are removed, a process known as developing. The resulting developed plate will contain a raised relief image in those areas where light passed through the negative. The flexible relief plate is wrapped around a cylinder on a printing press, and used to transfer ink onto a substrate, such as paper, film, bags, etc.
While many organic solvents are capable of dissolving non-crosslinked material from the plate, only a few are considered good developer solvents. This is because most solvents that dissolve the non-crosslinked material also absorb into the crosslinked portions of the plate, causing these areas to soften and swell, thus changing the shape of the plate, and swelling encourages erosion of the relief image by the brush in the developer tank. This erosion results in the final image on the plate becoming different from the target image on the negative. Hence printing quality suffers. The ideal developer solvent would dissolve and remove the non-crosslinked portions of the plate while at the same time not softening or swelling the crosslinked portions.
The general process of developing flexographic plates is well-known and described in detail in a number of U.S. Patents including Miura (U.S. Pat. No. 4,267,260), Shimizu (U.S. Pat. No. 4,271,261), Merrem (U.S. Pat. No. 4,539,288), Brault (U.S. Pat. No. 4,665,009), Hoffmann (U.S. Pat. No. 4,806,452), Kobayashi (U.S. Pat. No. 4,844,832), Worns (U.S. Pat. No. 4,847,182), Telser (U.S. Pat. No. 5,061,606), Frass (U.S. Pat. No. 5,077,177), Frass (U.S. Pat. No. 5,116,720), Telser (U.S. Pat. No. 5,128,234), Telser (U.S. Pat. No. 5,176,986), Larimer (U.S. Pat. No. 5,204,227), Telser (U.S. Pat. No. 5,240,815), Bach (U.S. Pat. No. 5,252,432), Schlosser (U.S. Pat. No. 5,312,719), Schober (U.S. Pat. No. 5,354,645), and Takagi (U.S. Pat. No. 5,578,420). However, the process or solvents used in these patents all suffer from numerous drawbacks, as explained below.
As Telser discusses in U.S. Pat. No. 5,061,606 most modern photopolymer printing plates generally have a multilayer structure. Typically the topmost layer is composed of a thin (5 micrometers in thickness) layer of polyamide referred to as the "cover layer" that serves to protect the delicate photopolymer during handling and exposure. Because this layer is resistant to some photopolymer developer solvents, an additive co-solvent is often incorporated into the developing solution to facilitate removal of this cover layer in the developing bath. Because polyamide is soluble in most organic alcohols, developing solutions typically contain a low percentage (10-30%) of an organic alcohol for this purpose.
Originally, exposed photopolymer plates were developed with a solvent mixture consisting of perchloroethylene to dissolve uncured photopolymer and butanol to remove the protective cover layer. However, this solvent mixture is toxic and dangerous, and therefore is no longer used (perchloroethylene is widely recognized to be carcinogenic, and butanol is flammable). A number of replacement solvents have been proposed to replace perc/butanol, but virtually all known mixtures have serious disadvantages.
For instance, in U.S. Pat. No. 4,267,260 Miura disclosed as early as 1981 that glycol ethers, DMSO, NMP, diethylformamide, xylene, cyclohexane, and monochlorobenzene can be used to develop plates, but they cause the plates to "swell" resulting in poor image quality during printing. In addition, diethylformamide (flash point 60.degree. C.), xylene (flash point 25.degree. C.) and cyclohexane (flash point -18.degree. C.) are flammable, making them dangerous or expensive to use, and DMSO has an intensely strong odor that is offensive. Further, glycol ethers are only effective on certain plate types, and therefore are restricted to a very small portion of the industry. In U.S. Pat. No. 4,271,261 Shimizu teaches the use of glycol ethers in combination with acids and/or gamma-butyrolactone (GBL). However, acids are corrosive and therefore attack machine parts, and are severe skin and eye irritants. Further, gamma-butyrolactone is not effective on many plate types. In U.S. Pat. No. 4,806,452 Hoffmann teaches the use of terpene hydrocarbons such as d-limonene in developer solvents, but terpene hydrocarbons have intense odors, are moderate skin and eye irritants, and have flash points below 141.degree. F., making the waste solvent mixture "hazardous" by RCRA guidelines. This leads to increased disposal cost and high regulatory compliance costs. Other inventors discuss the use of chlorinated solvents (which are toxic to people or are dangerous to the environment), mixed low-molecular-weight aromatic solvents (which are odoriferous and relatively toxic), and ketones (which are overly aggressive and odiferous). All of these solvents cause excessive plate swelling and/or cause plates to delaminate (come apart) during developing, and therefore are not useful. Low toxicity saturated hydrocarbons are also discussed, but they exhibit very poor solvency and therefore cannot process plates quickly enough. In U.S. Pat. No. 4,847,182 Worns discloses the use of d-limonene in combination with water, benzyl alcohol, and/or butanol as a developer solvent, but this invention suffers from the strong odor of d-limonene, as well as the low flash point of d-limonene that makes the waste solvent a RCRA hazardous waste. In U.S. Pat. No. 5,061,606 Telser discloses the use of hydrogenated petroleum fractions in combination with low levels of alcohols (to remove the cover layer) and monoterpenes (added as odorants), but this combination has poor solvency and therefore develops conventional plates much too slowly to be practical. In U.S. Pat. No. 5,077,177 Frass discloses the use of phenyl ethers as developers, but these solvents also have very strong odors which make them undesirable, and they generally have flash points too low to be practical (&lt;141.degree. F.). Further, Frass explains that d-limonene has the additional deficiency of not being able to develop plates containing nitrile rubber. In U.S. Pat. No. 5,312,719 Schlosser discloses the use of mixed aromatic solvents in combination with butanol and 2-ethyl butanol. Unfortunately, these mixed aromatic solvents are relatively toxic and distinctly odoriferous, and both butanol and 2-ethyl butanol are very odoriferous and have flash points below 141.degree. F., making the waste solvent hazardous. Therefore this combination is impractical. Finally, in U.S. Pat. No. 5,578,420 Takagi teaches the use of a 3-part solvent blend containing (A) mixed aromatic solvents to dissolve the photopolymer, (B) an alcohol to remove the protective "anti-tack resin layer" or cover layer, and (C) isobutyl isobutyrate as an odor masking agent. This invention is typical of prior art inventions in that it suffers from several common problems:
1. The developer solvent contains 25-70% of mixed aromatic solvents such as Solvesso 150 (product of Exxon Chemical Japan Limited) that have high odor intensity and moderate toxicity. These solvents are so odoriferous that an odor masking agent is required; PA1 2. The mixed low molecular weight aromatic solvents are so aggressive (dissolve the polymer so quickly) that concentrations above 70% excessively swell the photopolymer plate. For this reason a third component, that acts as a diluent, is added to "control the swelling action" of the active ingredient; PA1 3. The developer solvent is a non-azeotropic blend of three components, all with different boiling points. Because spent solvent is always reclaimed by distillation in which components preferentially vaporize in boiling point order, the distilled solvent can easily end up being depleted of one or more critical components. For this reason it is desirable to have a solvent system with as few components as possible, preferably no more than two, or to have a solvent system which is azeotropic; PA1 4. The so-called odor masking agent, isobutyl isobutyrate, has a flash point of only 99.degree. F., classifying it as a flammable liquid. Addition of significant amounts of this solvent to the blend will reduce the flash point of the mixture below 141.degree. F., causing the waste solvent to be classified as a RCRA hazardous waste. And although isobutyl isobutyrate is described as an odor-masking agent, it actually has a strong, unpleasant odor itself. PA1 (a) to provide a solvent that develops a wide range of solvent-developed photopolymer plates; PA1 (b) to provide a solvent that has a flash point in excess of 141.degree. F., so that the waste can be classified as non-hazardous after use; PA1 (c) to provide a solvent that minimizes unwanted plate swelling in the developer bath; PA1 (d) to provide a solvent that quickly dissolves uncured polymer but leaves the cured polymer intact; PA1 (e) to provide a solvent that has low toxicity so as not to pose a threat to users; PA1 (f) to provide a solvent that is environmentally safe, specifically a solvent that does not deplete the ozone layer or have an excessively long atmospheric lifetime; PA1 (g) to provide a solvent that does not contain components listed as toxic or acutely toxic on the EPA Sara Title 3 Section 313 list of toxic substances; PA1 (h) to provide a solvent that does not contain components that are classified as carcinogens, mutagens, or reproductive toxicants under California Proposition 65; PA1 (i) to provide a solvent that is temperature-stable during distillation, so that minimal chemical decomposition occurs during reprocessing; PA1 (j) to provide a solvent that does not contain components that are classified as hazardous air pollutants under the Federal Clean Air Act Amendments; PA1 (k) to provide a solvent that has low vapor pressure, and therefore produces low VOC emissions, but, at the same time, evaporates quickly from developed plates; PA1 (l) to provide a solvent that does not require adjustment after distillation (some multi-component mixtures require balancing after distillation because one or more components preferentially distill); PA1 (m) to provide a solvent that is priced at a moderate level, so as to contribute to the overall economy of the flexographic printing process; PA1 (n) to provide a solvent that has a very mild odor and therefore does not require an odor-masking agent; PA1 (o) to provide a solvent that has a lower boiling point than the boiling point of its constituents to facilitate reprocessing; PA1 (p) to provide a solvent that develops plates as rapidly as possible; PA1 (q) to provide a solvent that dries as rapidly as possible after plate development; PA1 (r) to provide a solvent that exhibits selective solvency for non-crosslinked photopolymer vs. crosslinked photopolymer; PA1 (s) to provide a solvent that is a blend of components that forms a true azeotrope so that the composition does not change upon distillation.